1. Field of the Invention
The present invention relates generally to an apparatus for controlling a power supply to a glow plug for a vehicular Diesel engine, and more particularly to an apparatus for controlling the power supply to the glow plug located within a corresponding combustion chamber or the glow plug located within an intake air passage of a vehicular internal combustion engine. Circuitry for halting a power supply to the glow plug is provided together with circutry for supplying a power supply current to the glow plug with an engine key inserted into a key hole of an engine key switch apparatus so that power cables for these circuitry are safely connected to a power supply.
2. Description of the Prior Art
FIG. 1 shows a conventional apparatus for controlling a power supply to a glow plug in a vehicular Diesel engine in order to preheat the glow plug at an earlier stage than usual so as to facilitate engine start. The conventional apparatus shown in FIG. 1 is disclosed by Japanese Patent Application Unexamined Open No. S55-69760, published on May 26, 1980.
A DC power supply (hereinafter referred to as a battery) 1 is connected to a glow plug 5 via contacts or a relay switch 4. An electromagnetic relay coil of the relay switch 4 closes the contacts of the relay switch 4 when energized in response to a signal from a control unit 3. The control unit 3 receives a voltage signal from the battery 1 via a manually operated ignition switch 2, and outputs the signal for a predetermined period of time. Therefore, the glow plug 5 receives the voltage from the battery 1 for the predetermined period of time upon closure of the ignition switch 2. In addition to the above-described construction, a relay switch 6 is connected in parallel with the ignition switch 2. When the relay switch 6 is turned on in response to an actuation of a timer 8 which in turn is actuated for a predetermined period of time t when a key-in switch 7 is turned on, the control unit 3 energizes the electromagnetic relay coil of the relay switch 4 so that the glow plug 5 receives the voltage from the battery 1 via the relay switch 4.
In this way, with an engine key inserted in an engine key switch apparatus having such switches as a key lock position, accessory position, on switch position, and ignition switch position, the key-in switch 7 is turned on so that the control unit 3 can be actuated to start preheating of the glow plug 5 with the ignition switch 2 not yet turned on.
The glow plug power supply control circuit having the glow plug power supply line denoted by symbol A supplies the voltage from the battery 1 to the glow plug 5 with the key-in switch 7 turned on and has an advantage of improving an engine starting characteristic due to an earlier preheating of the glow plug 5. However, the following disadvantages rises since there is no safety facility in the above-described apparatus when the battery 1 is replaced or power cables are installed to form the circuitry shown in FIG. 1 with the engine key inserted in the above-described engine key switch apparatus.
That is to say, since the power supply current flows into the timer 8 with the key-in switch 7 turned on, the timer 8 is actuated to turn the relay switch 6 on so that the glow plug 5 receives the voltage from the battery 1 by means of the control unit 3.
Hence, when both terminals of the battery 1 are connected to the power cables leading to the key-in switch 7, ignition switch 2, relay switch 4, and ground, respectively, a large current will instantaneously flow into the connection portions of the above-described electrical parts during connection and disconnection operations causing a large spark will at one of the cable connection portions. Consequently, cable terminals and power supply output terminals are often damaged and secondary failures such as electric circuit-shortings often occur due to touching on mutual electrical circuits or direct grounding of a portion to which a positive voltage is applied by an operator's hand or metallic tool.